The present invention relates to a process of manufacture and the product made therefrom. It relates to shaping sheet metal blanks, particularly to operations in which a sheet metal member having a continuous flange is folded along a bend line transverse to the flange, creating a continuous corner flange. As an example, the present invention is useful in forming a joist hanger having a seat member, a back member connected thereto, and parallel continuous corner flanges also connecting the two members.
When a sheet metal member having one or more flanges is bent transversely to the flange or flanges such that the flanges are swung inwardly on themselves, surplus material in the flanges along and near the bend develops. This excess material wants to fold over itself or crimp.
The problems caused by excess material are particularly acute in forming and wiping dies, operating at high frequencies. For example, if the tolerances in the press are such that the flange is closely contained between the punch and the die of the press, then this surplus material in the flange can fold over itself and tear, having no where into which to flow or bend. This condition is also known as wrinkling. In such presses, the material can also become caught, causing other portions of the sheet metal member to stretch.
Wrinkling is unsightly and is often perceived as a weak spot in the part. Stretching can also create undesirable indentations in the corner of the flange. Both wrinkling and stretching are not easily controlled between parts, such that there is a lack of uniformity between the parts. The present invention seeks to create a part, having a continuous corner flange that is smooth and lacks any visible wrinkles, crimps, puckering in the portions of the flange that have been compressed, and any deleterious stretching in the portions of the part near the compressed areas of the flange. The process creates more uniform parts.
The difficulties caused by the surplus material that develops can also prematurely age the press. It is known in the prior art that excess material that develops between components of a die can cause "technical difficulties" which affects the operation of the press. See U.S. Pat. No. 1,343,647, granted to R. S. Smith in 1920 at page 1, line 37.
There are a number of ways of avoiding the difficulties associated with forming a continuous corner flange on a high-speed wiping or forming die. For example, if the transverse bend is made with a large radius, the compressed area is reduced, and tearing is less likely. Further, reducing the height of the flange or flanges also reduces the portions of the flanges to be compressed, reducing the likelihood that tearing will occur.
However, it is often desirable to form a continuous corner flange in a part having a tight-radius, 90-degree bend with relatively high side flanges. For example, the inventor, using the present method can consistently form a tight-radius, 90-degree bend with a 9/16" high continuous corner flange without wrinkling, and with only minimal stretching the material in both 12 gauge and 18 gauge material. The height of the side flange is measured away from the transverse bend line and from the back along the proximal side of the flange to the top of the flange. With regard to stretching of the material, the inventor tested parts formed in a v-shaped forming die with openings in their side flanges and without openings in their side flanges. The inventor found that with an 18 gauge part with no openings in the side flanges, the material in the very corner of the part where the transverse bend line meets the flange bend line the part was thinned by 0.08". In comparison, the inventors noticed less thinning in the same part formed with a 1/8" diameter opening located on the transverse bend line in the side flange, the opening being adjacent the edge of the curved portion in the flange that results from the transverse and flange bends. The material thinned only 0.04" in the very corner. Thinning was similarly less in side-by-side comparisons of a parts formed with 12 gauge material.
There are prior art methods of forming similar shapes in light gauge sheet metal; however, they suffer from various drawbacks. For example, forming a short-radius, 90-degree bend with a continuous corner flange could be accomplished in a draw-action die. However, draw-action dies are relatively slow compared to forming or wiping dies. Also draw action dies require excess material around the part to hold the part while it is being stretched. This excess material usually needs to be cut off the part once the drawing operation is complete, adding an extra step to the process.
A number of other patented prior art processes form a transverse bend in a sheet metal part having corner flanges, however, these methods remove most or all of the material from the side flanges that is likely to be compressed. This avoids the problem of having the excess material of the flanges near the transverse bend crimp or wrinkle on itself; however, the flange is substantially weakened by the removal of most or all of the material near the transverse bend. See U.S. Pat. No. 1,925,804, granted to William C. Hiering on Sep. 5, 1933, and U.S. Pat. No. 5,203,069, granted to Kurt Hennig on Apr. 20, 1992.
The present inventive method for laterally bending an elongated sheet metal member having one or more side flanges treats the above problems, preventing tearing or undue stretching of the metal of and around the flanges without removing excessive amounts of material from the flanges near the transverse bend, or using a slower draw method, or reducing the height of the flanges or reducing the sharpness of the transverse bend.
The method of the present invention is particularly suited for forming certain types of sheet metal joist hangers. Sheet metal joist hangers are widely used in wood frame construction to attach joists to carrying members. The method of the present invention has particular relevance for forming light gauge hangers having an upright back with parallel opposed side flanges extending therefrom, and a flanged horizontal seat extending outwardly from the back in the same direction as the flanges to form a bearing area for a joist. See U.S. Pat. No. 4,802,786 granted to James G. Yauger and John M. Rushton on Feb. 7, 1989 for an example of such a hanger. U.S. Pat. No. 3,633,950, granted to Tyrell T. Gilb on Jan. 11, 1972 is also exemplary.
The particular joist hangers described in the two above-identified patents are used in the panelized roofing industry, where large roofs are normal, requiring the use of hundreds of such hangers at a time. Currently, many industrial buildings are designed with panelized roofs, creating a high demand for such hangers. The present invention provides an economic method for forming the critical transverse bend in such hangers between the back member and the seat.